1. Field of the Invention
The present invention relates to a sensor for detecting movements of muscle or body tissue, in particular a sensor for an implantable heart stimulator, by means of which the contraction of a heart can be measured and/or determined.
2. Description of the Prior Art
In situations where an implanted stimulator is used, it is normally important to detect the muscular contraction of the stimulated muscle. For example, in the case of a pacemaker the heart contraction is of special interest. Heart contractions may generally have different origins and hence differ significantly from each other as to their characteristic shape or dependence of time. They may be normal, thus triggered by the internal conduction or xe2x80x9cnervousxe2x80x9d or purkinje system of the heart, stimulated or of type extra contraction (extra systole; VES, SVES or PVC). In sensing heart contractions by means of electrical measurements stimulation polarization potentials and muscular movement interference can create noise virtually making the measurements impossible. In these cases, a device sensitive to the mechanical contraction of the contraction of the stimulated, would confirm and improve the function of a heart stimulator.
In some situations it would be of interest to know also the efficiency of the stimulation or the direction of the stimulation propagation in order to improve the stimulation device or method, such as improving the stimulation algorithm. Examples of such cases are where a differentiation is wanted or required between conducted (normal) contraction and stimulated contraction of the heart (Autocapture), where a differentiation is wanted between normal conducted contraction and extra systole and where ischemia is to be detected. In those cases the change of contraction propagation in the heart muscle will be different, since the propagation pattern or the direction of the contraction movement will be different. For example, in cardiac ischemia, a portion of the heart wall or heart muscle suffers from an insufficient oxygen supply. This means that a propagating depolarization and thus the contraction cannot pass this portion or at least is subject to a delayed conduction and thus contraction. As a result, alternative propagation paths are favored, causing a change in the movement pattern. In order to detect such types of pathological changes so that they can be taken into account when stimulating or monitoring the heart function, a device is needed which is capable of sensing differences in the movement pattern.
Known methods of sensing heart contraction include:
Intracardiac Electrographic Monitoring (IEGM), which does not always reflect the actual contraction activity as already indicated above, owing to the electrical measurement made. The measurement can thus be disturbed by other electrical signals such as from muscular activity and polarization. Direction, changes of movement pattern or generally the character of the contraction cannot be determined. The measurement only provides information about the changes that occur in a close vicinity of the site of the sensing electrode.
Systolic pressure sensing gives information on the contraction and possibly also the efficiency thereof. It is usually not significantly affected by other body movements and is not affected by electrical signals. However, it cannot detect the propagation direction or changes in the movement pattern.
Measuring by means of an accelerometer. This method senses only contraction forces thus does not sense the propagation direction or changes in the movement pattern.
Measuring using ultrasonic waves. No implantable device is available. The current consumption for making the measurements is too high.
Measuring the impedance of the heart. This could be done for example between an electrode and the pacemaker housing. However, it can sense only changes of the heart volume. In U.S. Pat. No. 5,514,171 a pressure and heart movement sensor of the systolic pressure sensing type is disclosed. An electric quantity is measured between two electrical conductors in the electrode cable. In one embodiment, see FIG. 2, a sensor medium such as an electrolyte is enclosed in an elongated cavity having elastic walls. The cavity is formed by a widened portion of an elastic tube, which in its other portions has a smaller diameter only enclosing a centrally disposed electrical conductor. The impedance is measured between two conductive plates located at each end of the cavity, the impedance varying in dependence of the pressure of the ambient medium where the sensor cavity is placed. It is not described how this sensor cavity and associated conductors can be incorporated in an electrode lead for a heart stimulator. Other systolic pressure sensors are disclosed in U.S. Pat. Nos. 4,784,151, 4,924,872 and 4,600,017.
It is an object of the invention to provide a sensing device that can be incorporated in an electrode lead for an implantable stimulator and is capable of detecting movements of the surrounding tissue where it is placed, in particular movements of the heart muscle.
It is another object of the invention to provide a sensing device for sensing tissue movements that is not affected by the environment or any electrical phenomena in the tissue.
It is another object of the invention to provide a sensing device that is capable of sensing the orientation of movements of the surrounding, where the sensing device is placed, in particular the orientation of a contraction caused by a depolarization propagating in a muscle such as the heart muscle.
The problem that is solved by the invention is thus to provide a sensor that can be built into for example an electrode lead for a heart stimulator, that is substantially unaffected by electrical signals from muscle activity and signals originating from stimulation and that can detect movements of body tissue, in particular the orientation of movements and changes of movement patterns.
The above object is achieved in accordance with the principles of the present invention in an arragement having a deformation sensor for measuring or detecting the bending of an elongated body in which the deformation sensor is placed, the body preferably being an electrode lead which is part of an implantable system adapted for muscular or neuron stimulation in a living being, in particular a heart stimulator, the body thus e.g. being an electrode lead included in the electrode system of a heart stimulator. In practice, the tip end of such an electrode lead is positioned in contact with the muscle to be stimulated, i.e. in the preferred case with the heart. The end portion of the lead is placed to have an arched or curved configuration so that the tip end will maintain its contact with the tissue in order to coop with growth, movements and stretching of the living being in which the electrode lead is arranged. When the muscle contracts and thus is becoming thicker, the bending radius of the lead end portion will be lower and thus the curvature thereof larger.
The deformation sensor can be any known type capable of providing some electrical signal responding to a bending movement of the sensor and of the material to which the sensor is attached. For example, it is possible to use a device containing an electrically conducting material that is arranged so that it changes its electrical characteristics when it is subjected to bending. One such device contain resistance elements that changes their resistance when deformed, such as strain gauges. The resistance element is then attached to some suitable flexible body that follows the movement of the medium or tissue where it is placed.
By attaching two such detector elements in one electrode lead at asymmetrical positions in the lead, in addition the orientation of the bending movement can be sensed. Since a lead normally is elongated and/or cylindrical, the term asymmetrical for the case of two detector element means that the detector elements are not placed at diametrically opposed locations, i.e. not in the same plane extending through the longitudinal axis of the body. Preferably, they are instead located in planes through the longitudinal axis, which form an angle of 30xc2x0-150xc2x0 to each other, the angle preferably being essentially equal to 90xc2x0. The detector elements should also be located at or in the same longitudinal region of the lead in order to provide information on the bending movement. Such an arrangement of two mechanical detector elements can thus be generally used in order to obtain valuable information on bending directions. In the case where more than two detector elements are used, at least one pair of detector elements should have the asymmetrical position as described above.
A resistor element used as the deformation sensor is formed by a channel in the lead body, extending in the longitudinal direction thereof. The body can be an electrically isolating sleeve surrounding a central electrical conductor and then the channel is located in the material of the sleeve, the sleeve having the same uniform inner and outer diameter also at the region where the channel is made. The channel is filled with an electrically conductive fluid, such as a suitable electrolyte, e.g. a salt diluted in water, preferably a saline solution being biologically inert to body fluid. Terminals are electrically connected to the ends of the channel for connection to circuitry for the resistance of the fluid between the ends of the channel. The fluid is thus located in a closed cavity. When the lead is subjected to a bending movement, the cavity changes its shape, in particular its diameter, and then also the resistance of the enclosed fluid changes. Such a device could generally also be sensitive to the ambient pressure, see U.S. Pat. No. 5,514,171 discussed above, but by using a substantially incompressible fluid, i.e. a fluid which does not change its volume significantly when subjected to the pressures existing at the place where the sensor is intended to be used, a change of the ambient pressure will not influence the resistance of the enclosed amount of fluid.